To reduce harmful substance contained in exhaust gases discharged from the engines of automobiles, etc., exhaust-gas-cleaning, catalytic converters and particulate-matter-capturing filters comprising ceramic honeycomb structures are used. Usually, the ceramic honeycomb structure comprises a peripheral wall, and a large number of cells defined by orthogonal cell walls inside the peripheral wall.
The ceramic honeycomb structure is produced, for example, by blending cordierite-forming material powders, a binder, and if necessary, molding aids such as a pore-forming material, a dispersant, a lubricant, etc. with water, supplying the resultant plasticizable molding material to a material-flowing path of a molding apparatus, pushing the molding material toward honeycomb grooves disposed at one end of the material-flowing path by a plunger, etc., extrusion-molding the molding material through the honeycomb grooves, cutting the extrudate to a predetermined length to obtain a honeycomb molding, and drying and sintering it.
The molding die comprises a large number of material-supplying holes disposed on the upstream side, and molding grooves disposed on the downstream side for communicating with the material-supplying holes. Because the material-supplying holes and the molding grooves have extremely fine structures, foreign matter, insufficiently blended starting material bulks, etc. contained in the molding material may be trapped in the material-supplying holes or the molding grooves in the course of extruding the molding material through the molding die. As a result, an uneven molding speed, the bending of moldings, and the clogging of the material-supplying holes or the molding grooves occur, resulting in partially defective cell walls (broken cells). Broken cells provide ceramic honeycomb structures with insufficient strength, resulting in breakage during use, and low particulate-matter-capturing efficiency when they are used as particulate-matter-capturing filters. To remove foreign matter from the molding material, improve the uniformity of the starting material, or to prevent the moldings from bending, the following technologies have been disclosed.
JP 60-45564 B discloses a wet ceramic extrusion-molding apparatus comprising a breaker plate (porous plate) between a screw tip and a die, the breaker plate holding a screen assembly comprising at least three pairs of coarse screens and fine screens laminated alternately, the screen assembly being arranged such that a coarse screen is opposite to the breaker plate, and other fine screens than that constituting a pair with the coarse screen opposing the breaker plate being provided with notch holes. JP 60-45564 B describes that the notch holes increase the effective areas of fine screens, thereby increasing the usable time period of filters (screens).
JP 6-807 A discloses a molding apparatus comprising a wear-resistant metal net screen disposed between an auger screw and an orifice on its downstream side, and a dividing plate (porous plate) having a large number of holes and disposed downstream of the screen for reinforcing the screen. JP 6-807 A describes that because the wear-resistant metal net screen removes a hard material and lamination, moldings free from sagging and breakage during sintering can be obtained.
However, while extrusion-molding the molding material in the molding apparatuses described in JP 60-45564 B and JP 6-807 A, a portion of the screen covering the penetrating holes of the porous plate is subject to an excess load, so that screen-constituting wires are expanded by repeated molding, providing the screen with large openings, and sometimes breaking it. The expansion of screen openings and the breakage of screens make it less effective to remove foreign matter and insufficiently blended starting material bulks from the molding material, providing the extruded moldings with bent or broken cells.
Also, when a plunger-type extrusion-molding apparatus is used, the molding material is not continuously but batchwise supplied to a material-flowing path one after another. Accordingly, the plunger should be retreated to the upstream side upon completion of each step. When the plunger is retreated to the upstream side, a molding material 171 remaining in the material-flowing path moves upstream with the plunger 16 as shown in FIG. 5(a). Accordingly, the screen 15 may peel from the porous plate 14 in the structures of porous plates and screens described in JP 60-45564 B and JP 6-807 A. Particularly in the case of a molding material comprising ceramic materials and a foaming or foamed resin as a pore-forming material, the resin released from the extrusion pressure expands when the plunger retreats upstream, pushing the molding material remaining in the material-flowing path upstream, so that the screen easily peels from the porous plate.
When the next batch of a molding material 17 is charged into the material-flowing path and molded, the screen 15 peeling from the porous plate 14 is bent, resulting in a bent portion 152, and a portion 142 free from the screen 15, as shown in FIG. 5(b). The molding material has a high flow rate in the portion 142 free from the screen 15, while it has a low flow rate in the bent portion 152 of the screen 15, resulting in bent ceramic moldings. Further, in the portion 142 free from the screen 15, moldings are contaminated with foreign matter and insufficiently blended starting material blocks in the starting material, and foreign matter clogs the die, likely resulting in honeycomb moldings with broken cells.
Although the pores 141 of the porous plate 14 can be fixed to the screen 15 with wires 20, for instance, as shown in FIG. 5(c), lest that a center portion of the screen peels from the porous plate when the plunger returns to the upstream side after the completion of one-batch molding, the wires may be broken by wearing after repeated molding, with their debris mixed into moldings and clogging the die, resulting in honeycomb moldings with broken cells. Further, with the wires broken by wearing, the screen peels from the porous plate when the plunger returns to the upstream side after the completion of one-batch molding, resulting in the deformed portions 152 of the screen 15 and portions 142 free from the screen 15, as shown in FIG. 5(b). Thus, the resultant ceramic moldings are bent and have broken cells, as described above.
JP 2008-137186 A discloses an extrusion-molding apparatus comprising an extrusion screw means having an extrusion screw in a barrel for blending and advancing a ceramic material, and a molding die connected to a tip end of the extrusion screw means via a resistance pipe; a ceramic material pushed into the resistance pipe from the extrusion screw means being extruded from the molding die to form a ceramic molding having a desired shape; a rectifying plate (porous plate) having pluralities of openings for passing the ceramic material being disposed between the extrusion screw means and the resistance pipe; the rectifying plate having pluralities of regions; and at least one region having a different opening ratio from those of the other regions. It also discloses the adjustment of the opening ratio by laminating a rectifying plate (A) having the same opening ratio in all regions with a rectifying plate (B), ¾ of which is cut off to provide a penetrating hole, and the overlapping of the openings being changed by making their phases out of alignment. JP 2008-137186 A describes that the rectifying plate has different opening ratios from region to region, to adjust the flow rate of the ceramic material passing through the regions of the rectifying plate, thereby suppressing unevenness in the flow rate of the ceramic material in the extrusion-molding apparatus, and the bending of the resultant ceramic moldings.
However, although the molding apparatus described in JP 2008-137186 A comprises a porous plate having adjusted opening ratios in regions for suppressing the bending of moldings, it does not comprise a screen, so that the moldings are contaminated with foreign matter and insufficiently blended starting material bulks in the starting material, and that foreign matter clogs the die, resulting in honeycomb moldings with broken cells. In the molding apparatus described in JP 2008-137186 A, when a screen is disposed upstream of two rectifying plates (A), (B) to prevent the moldings from being contaminated with foreign matter and insufficiently blended starting material bulks in the starting material, as described in JP 60-45564 B and JP 6-807 A, an excess load is applied to part of the screen covering the penetrating holes of the rectifying plate, so that wires are elongated by repeated molding to expand the openings of the screen, and that the screen may be broken. If the screen has expanded openings or is broken, its function of removing foreign matter and insufficiently blended starting material bulks in the molding material is reduced, resulting in bent moldings and those with broken cells.